The use of transparent conducting oxides (TCOs) as the basis for all-transparent electronic components has been gaining considerable interest as they point to a pivotal change in both scientific and consumer electronics. The reason TCOs1-3 will undoubtedly lead, or play a primary role, in this new exciting field is that, when degenerately doped, they exhibit near-metallic conductivities while maintaining bandgaps in the 3.6-3.8 eV4. Transparent conducting oxides were first discovered in 1907 by Badeker; he discovered that by oxidizing a Cd metal film during deposition, he could create a material that was both transparent and electrically conducting5. Yet, although CdO was the first TCO discovered, it is tin-doped indium oxide, commonly known as “ITO”, that became the most widely used commercial TCO in optoelectronics. In the 1990s6, research groups began developing multi-cation systems such as Gd-doped InOx, F-doped In2O35; and Al-, In-, B- Ga-doped ZnO3, 5. Ternary systems also exist, which include (Zn—In—Ga)—O, (Ga—In—Sn)—O, and (Cd—In—Sn)—O6. There are many applications of TCOs that include their use as a transparent anode in optoelectronics5, 7-10, as heat mirrors for solar collectors11-13, as the antireflection coating and/or electrodes in photovoltaics12, 14, 15, photodetectors15, flat panel and crystal displays16, de-icers, and IR reflective coatings for building windows that reduce heating and cooling costs17, 18.
The possibility of truly realizing invisible electronics19-21 seems to be eminent as transistor action within an all-TCO framework has already been achieved. Yet, in addition to transistors, transparent electronic circuits will require the development of transparent interconnects—at or near the nanoscale. To this end, patterning of TCO blanket films via chemical and physical means has been reported22-26. In particular, TCO patterning aiming to create transparent conducting wires has been shown with wet etching through a resist mask26. The advantages to user-defined patterns via large-scale, parallel processing are clear. Yet one distinct difficulty remains, namely insufficient control of the reaction due to its dependence on local microstructure, which limits the pattern resolution to a few hundred nanometers. Another example of attempts to create nanoscale wires in predetermined patterns, although not of TCOs, is the use of ion beams (e.g. B+, O+, P+, or As+ beams at 5×1014-1016 ions cm−2 in the accelerating voltage range of 35-100 keV) to locally decompose metal-organic blanket thin films. This process has been shown to create 250 nm tall metallic structures with linewidths of ˜330 nm27, 28. While this technique does achieve impressive structures (certainly with metallic conductivities) they are nonetheless not transparent, the effect of which would dominate when the necessary number of interconnect for viable electronics is considered. Thus it is evident that the need for TCO-based nanowires with precise positioning is necessary. To this end, the lithographic writing of nanoscale, optically transparent, electrically conducting wires that are embedded within a highly insulating undoped metal oxide host via focused ion beam (FIB) implantation was recently reported29 by our group.
The fabrication of these TCO wires was evidenced with a combination of advanced atomic force microscopy (AFM) techniques and secondary ion mass spectrometry (SIMS) depth profiling. The wire dimensions achieved were ≦160 nm wide and nominally 7 nm deep, with user-defined lengths. Equally important, state-of-the-art FIB processing implies full lithographic control which in turn leads to completely arbitrary shapes, and are theoretically limitless in length and connectivity (Sosa et al29). This doping technique—using a FIB beam to controllably implant Ga into undoped oxide host create patterns at the nanoscale—yields high contrast in electrical conductivity between doped and undoped material as evidenced in conductive AFM images, meanwhile achieving minimal damage to the surface (Sosa et. al.29).
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.